The present invention relates to a magneto-optical recording medium for recording/reproducing information with laser light by utilizing magneto-optical effects, and particularly relates to a magneto-optical recording medium in which each recording magnetic domain in a recording layer is extended and reproduced.
A number of studies and examinations have been made to promote high-densification in magneto-optical recording by utilizing magneto-optical effects to achieve repeatedly-overwritable information recording media with further greater capacity.
In such magneto-optical recording media, arises a drawback in that reproduction characteristics deteriorate as the diameter of recording bits as recording-use magnetic domains and the distance of the recording bits become smaller relative to the beam diameter of light beam converged on a medium.
This stems from that each recording bit cannot be reproduced separately, since a beam spot of the light beam converged on one target recording bit falls also on adjacent recording bits.
To solve the foregoing problem, the configuration of a recording medium, the reproducing method, etc. have been innovated to increase the recording density, and as a result, the super high resolution reproduction, the magnetic domain extending reproduction utilizing the moving (displacement) of magnetic domain walls, and the like are proposed. Here, the super high resolution reproduction and the magnetic domain extending reproduction will be explained below.
First, super high resolution technology is explained as a technology to achieve high densification of a magneto-optical recording medium.
As shown in FIG. 9, according to the magnetostatic-coupling-type super high resolution technology disclosed by, for example, the Japanese Publication for Laid-Open Patent Application No. 40600/1998 (Tokukaihei 10-40600, Date of Publication: Feb. 13, 1998) and the Japanese Publication for Laid-Open Patent Application No. 150418/1994 (Tokukaihei 6-150418, Date of Publication: May 31, 1994), a medium has at least a reproduction layer 121, a non-magnetic intermediate layer 112, and a recording layer 104 in this order. Arrows in each magnetic layer represent directions of magnetizations. The reproduction layer 121 is prepared so as to be in an in-plain magnetization at room temperature and to be turned to a perpendicular magnetization state when temperature rises to above a certain critical temperature during reproduction. The exchange-coupling between the reproduction layer 121 and the recording layer 104 is broken by the presence of the non-magnetic intermediate layer 112.
No magneto-optical signal is generated in the reproduction layer 121 at room temperature since the reproduction layer 121 is in an in-plain magnetization state, but during reproduction, heated by a projected beam, only a central part of the beam spot of the medium (a part behind the beam spot center in the moving direction of the beam spot when the beam spot moves at a high linear speed) exhibits a perpendicular magnetization, only from which magneto-optical signals are generated. Let the part be called xe2x80x9caperture,xe2x80x9d and the reproduction layer 121 is magnetostatically coupled with the recording layer 104 only at the aperture area in accordance with a magnetic field generated by a recording magnetic domain in the recording layer 104, so that a magnetization direction thereof is determined according to the recording magnetic domain. Therefore, by this scheme, a recording magnetic domain can be selected and read only from the area of the aperture, thereby allowing a structure of micro magnetic domains, or high densification, of the magneto-optical recording medium.
The foregoing scheme, however, has a drawback in that a reproduction signal quantity decreases as the recording magnetic domains become smaller in size.
On the other hand, the magnetic domain extending reproduction is a scheme of reproduction by extending small recording magnetic domains, which ensures that a reproduction signal quantity great enough can be obtained even with small recording magnetic domains. The following description will explain the foregoing scheme, while referring to FIG. 10.
As shown in FIG. 10, magnetic films in a multi-layer structure are exchange-coupled thereby causing small recording magnetic domains in a recording layer 104 to be extended in a magnetic domain extending layer (displacement layer) 102 so that amplitudes of reproduction signals are increased, thereby resulting in that high-density recording is realized. This is disclosed by the Japanese Publication for Laid-Open Patent Application No. 114750/1995 (Tokukaihei 7-114750, Date of Publication: May 2, 1995). Incidentally, arrows in the layers represent directions of sub-lattice magnetizations of transition metals in the film, and in each layer are formed magnetic domain walls (Bloch walls) 110, each being present between adjacent magnetic domains having magnetizations in opposite directions.
To realize the magnetic domain extending reproduction, the following requirements have to be satisfied.
1. The recording layer 104 stably maintains small magnetic domains in a temperature range from room temperature to a temperature for reproduction.
2. At least in the vicinity of a Curie temperature Tc3 of the intermediate layer 103, the recording layer 104, the intermediate layer 103, and the magnetic domain extending layer 102 are exchange-coupled.
3. The intermediate layer 103 loses magnetic order when the temperature becomes higher than the foregoing Curie temperature Tc3, thereby breaking the exchange-coupling from the recording layer 104 to the magnetic domain extending layer 102 in a temperature range above the Curie temperature Tc3.
4. At a region where the foregoing exchange-coupling is broken, a magnetic domain wall 110 moves (is displaced) relative to a domain transferred by the exchange-coupling, since the frictional force due to magnetic domain wall coercivity in the magnetic domain extending layer 102 is small and a magnetic domain wall energy gradient is generated by a temperature gradient. As a result, the magnetization in the foregoing region is directed in the same direction as that transferred by the exchange-coupling.
The following description will explain in more detail the magnetic domain extending reproduction disclosed in Tokukaihei 7-114750, while referring to FIG. 10.
The moving of the magnetic domain wall is explained first.
A graph in the center of FIG. 10 shows the temperature distribution in the center of a track when a laser is projected on the optical recording medium which shifts rightward relative to the laser. Here, since the disk as the recording medium moves at a high linear speed, a portion where the film temperature becomes highest appears behind a center of a beam spot in the moving direction of the beam spot relative to the disk.
Further, a graph below in FIG. 10 shows distribution of a magnetic domain wall energy density "sgr"2 in a radial direction in the magnetic domain extending layer 102. Normally, the magnetic domain wall energy density "sgr"2 decreases as the temperature rises, becoming 0 at the Curie temperature or above. Therefore, in the case of the temperature gradient in the radial direction as shown in the graph in the center of FIG. 10, the magnetic domain wall energy density "sgr"2 decreases to a level corresponding to the Curie temperature, as shown in the graph below in the same figure.
Here, a force F2 expressed by the following formula shown below is exerted to magnetic domain walls 110 in each layer at a position x in the radial direction:
F2=xe2x88x92d"sgr"2/dx
Since the force F2 is exerted so as to move the magnetic domain walls 110 toward where the magnetic domain wall energy is lower and the magnetic domain extending layer 102 has a smaller frictional force due to coercivity of the magnetic domain wall 110 compared with the other magnetic layers thereby having greater magnetic domain wall mobility, the magnetic domain wall therein is moved by the force F2 toward where the magnetic domain wall energy is lower, when the exchange-coupling force from the intermediate layer 103 is broken.
Next, the following description will explain the magnetic domain expansion.
In FIG. 10, in a portion on the disk before the laser light is projected, i.e., a portion at room temperature, the three magnetic layers are exchange-coupled, so that the magnetic domains recorded in the recording layer 104 are transferred to the magnetic domain extending layer 102. Here, the magnetic domain walls 110 are present in each layer, each being between adjacent magnetic domains having magnetizations of directions opposite to each other.
Where the film temperature is not lower than the Curie temperature Tc3 of the intermediate layer 103, the magnetization of the intermediate layer 103 disappears thereby breaking the exchange-coupling between the magnetic domain extending layer 102 and the recording layer 104. As a result, the magnetic domain extending layer 102 is no longer capable of keeping the magnetic domain wall 110, letting the magnetic domain wall 110 move to a high-temperature side according to the force F2 exerted to the magnetic domain wall 110. Here, the speed of movement of the magnetic domain wall 110 is sufficiently higher than the moving speed of the medium. Therefore, a larger magnetic domain than that recorded in the recording layer 104 is transferred to the magnetic domain extending layer 102. The extended magnetic domain is further transferred to the reproduction layer 121 by means of the exchange-coupling force.
The foregoing magnetic domain extending reproduction has the following drawbacks, however. Namely, in the magnetic domain extending reproduction, magnetic characteristics have to be adjusted, since magnetic domains are transferred to the reproduction layer 121 by exchange-coupling with the magnetic domain extending layer 102. Here, since transfer of a recording magnetic domain is carried out by exchange-coupling all the magnetic layers from the recording layer 104 to the reproduction layer 121, adjustment of magnetic characteristics of the respective layers becomes more difficult as the multi-film structure becomes complicated.
Besides, though the magnetic domain wall 110 has to be smoothly moved in a region in which the exchange-coupling of the magnetic domain extending layer 102 with the recording layer 104 through the intermediate layer 103 is broken, to satisfy both the requirements of the movement of the magnetic domain wall 110 and the exchange-coupling transfer to the reproduction layer 121 at the same time is very difficult.
Moreover, there arises a drawback in that the movement of the magnetic domain wall 110 is obstructed since an exchange force from the reproduction layer 121 is exerted to the magnetic domain extending layer 102.
Incidentally, in the aforementioned super high resolution medium of a magnetostatic coupling type (see FIG. 9), magnetization information of small recording magnetic domains are transferred to the reproduction layer 121 by magnetostatic coupling. In so doing, the magnetostatic coupling has to be realized by use of a very small magnetic field generated by a small recording magnetic domain, and therefore, it is necessary to provide the recording layer 104 and the reproduction layer 121 closest possible to each other unless becoming exchange-coupled. Therefore, the non-magnetic intermediate layer 112 is not allowed to be thick, and the thickness thereof is strictly limited in a range such that the exchange-coupling is broken while the magnetostatic coupling is realized.
The object of the present invention is to provide a magneto-optical recording medium in which smooth moving of magnetic domain wall and extension of magnetic domains are realized, and further, which is capable of increasing signal intensity with a simpler structure.
To achieve the foregoing object, a magneto-optical recording medium of the present invention is characterized in including (1) a recording layer in which a plurality of recording magnetic domains are formed, (2) a first exchange-coupling breaking layer for breaking exchange-coupling with the recording layer at a temperature not lower than a predetermined temperature, (3) a magnetic domain extending layer in which upon breaking of exchange-coupling with the recording layer, a magnetic domain wall moves to a higher-temperature side so that an extended magnetic domain is formed, and (4) a second exchange-coupling breaking layer for breaking exchange-coupling between the magnetic domain extending layer and a reproduction layer, while making the two layers magnetostatically coupled.
With the foregoing arrangement, when the first exchange-coupling breaking layer is at a temperature lower than the predetermined temperature, the recording layer and the magnetic domain extending layer are exchange-coupled, and recording magnetic domains in the recording layer are transferred to the magnetic domain extending layer. On the other hand, when being heated to a predetermined temperature or above, the exchange-coupling between the recording layer and the magnetic domain extending layer is broken by the first exchange-coupling breaking layer. The magnetic domain extending layer is thus no longer exchange-coupled with the recording layer, losing magnetic order. As a result, the magnetic domain extending layer becomes unable to maintain magnetic domain walls, letting the magnetic domain walls move to a higher temperature side from positions at a magnetic domain already transferred by the foregoing exchange-coupling. As a result, the magnetic domain transferred is extended to the higher temperature side, whereby an extended magnetic domain is formed.
The exchange-coupling between the magnetic domain extending layer and the reproduction layer is broken by the second exchange-coupling breading layer, while the reproduction layer and the magnetic domain extending layer are magnetostatically coupled. This magnetostatic coupling causes magnetic flux generated from the extended magnetic domain formed in the magnetic domain extending layer to reach the reproduction layer, causing the extended magnetic domain to be transferred to the reproduction layer.
With the foregoing magnetostatic coupling between the reproduction layer and the magnetic domain extending layer, restrictions on the magnetic characteristics in the transfer of the extended magnetic domains from the magnetic domain extending layer to the reproduction layer are eased. In other words, since magnetic domain walls in the magnetic domain extending layer are smoothly moved while never being affected by the exchange-coupling force from the reproduction layer, restrictions on material selection of each layer are eased. In addition, the reproduction layer is no longer subjected to the restrictions on the magnetic characteristics relating to the exchange-coupling, either, and therefore, a material having excellent reproduction signal characteristics can be selected as material for forming the reproduction layer.
Incidentally, in the conventional super high resolution medium of a magnetostatic coupling type wherein each small magnetic domain is transferred in substantially the same size by magnetostatic coupling and magnetic domains surrounding it have to be masked, a reproduction region in the reproduction layer has to be limited to a very small area. In contrast, in the present magneto-optical recording medium, since an extended magnetic domain in the magnetic domain extending layer is transferred to the reproduction layer, a reproduction region of the reproduction layer can be made larger, and a reproduction signal with a greater amplitude can be obtained therefrom.
Furthermore, in the conventional super high resolution medium of a magnetostatic coupling type, the recording layer and the reproduction layer have to be made as close as possible unless they become exchange-coupled, since magnetostatic coupling is realized by using a very small magnetic field generated by a small magnetic domain. In contrast, in the present magneto-optical recording medium, since a large magnetic field is generated from the extended magnetic domain in the magnetic domain extending layer, the restriction on the distance between the recording layer and the reproduction layer is remarkably eased, thereby enabling to form the second exchange-coupling breaking layer thicker to some extent. This further makes it possible to use both the Kerr effect and the Faraday effect of the reproduction layer, and even to obtain reproduction signals with further greater amplitudes, by optimizing multiple interference of light.
As described above, in the case of the present magneto-optical recording medium wherein extended magnetic domains are transferred to the reproduction layer and reproduction signals with great amplitudes are obtained, even with an increase in a linear recording density, reliable reproduction signals with sufficiently great amplitudes can be obtained, not being easily affected by noise.
The foregoing magneto-optical recording medium may be further arranged so that the reproduction layer and the magnetic domain extending layer are made of rare earth-transition metals, and that at least in a temperature range from room temperature to a Curie temperature of the magnetic domain extending layer, the reproduction layer and the magnetic domain extending layer have sub-lattice magnetizations with a same polarity.
According to the foregoing arrangement, since the reproduction layer and the magnetic domain extending layer are magnetostatically coupled, their magnetizations are directed in the same direction. Further, since the sub-lattice magnetizations in the foregoing two layers have the same polarity in the foregoing temperature range, the sub-lattice magnetizations of the layers are also directed in the same direction. With this, rotational angles of polarized lights obtained by three magneto-optical effects of the Kerr effect and the Faraday effect of the reproduction layer and the Kerr effect of the magnetic domain extending layer are directed in the same direction. As a result, a further greater rotational angle is obtained, and hence, reproduction signals with greater amplitudes can be obtained.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.